Combating Unscheduled Shutdowns and Outages by Utilising Non-Intrusive Inspection Technology

Combating Unscheduled Shutdowns and Outages by Utilising Non-Intrusive Inspection Technology

William R Sharp
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OVERVIEW

The requirement to combat the risk of unscheduled plant shutdowns and outages is of course obvious and is a direct result of these being a major cause of unplanned costs to plant operators. The ability to carry out inspection of plant non-intrusively and where possible on line can be an extremely important tool and can be used to achieve this goal. We will therefore review the ultrasonic techniques available and how they can be applied to demonstrate fitness for purpose within planned Plant Management Integrity and Condition Monitoring Systems. Other recent developments such as the move towards condition rather than time based inspection programs also necessitate the development of non-intrusive inspection techniques. All the methods described in this paper can de used Non-Intrusively and in most instances outside of plant shutdowns.

1. INTRODUCTION

It is common knowledge that the majority of industrial plant around the world is subject to licensing by a statutory body or insurance by major underwriters. The norm has been for time limits to be set on the operation of the plant. The operational time limits are established by using data gained from in-service inspection, which would in general be based on a planned maintenance program. Inspection techniques have historically been based on conventional non-destructive testing methods that can be subjective and may be prone to operator error. It is also a fact that most conventional NDT methods have to be applied when the plant is off line during either planned of forced shut downs. The development of non-intrusive inspection techniques has enabled major operators of capital-intensive plant to move from a time based inspection strategy to one that is condition based.

2. TECHNOLOGY AVAILABLE FOR CRITICAL PLANT ASSESSMENT

Ultrasonic imaging techniques can provide the information needed for plant integrity assessment. The single most important benefit of ultrasonic imaging techniques is the improvements they offer in controlling coverage and ensuring high quality usable data and the advantage of gathering detailed information of plant integrity prior to planned outages. Over and above this is the fact that these methods can be applied non-intrusively. The equipment and services are available internationally from COOPERHEAT and have been successfully applied in the field for many years. They can range from very simple scanning frames to fully programmable automated scanners, and include sophisticated processor controlled colour graphic display formats with data storage, manipulation and display capabilities. COOPERHEAT have, based on many years experience in the field, selected the latest generation ultrasonic inspection system the Technology Design Pocket Scan.
In order to ensure complete coverage of a vessel or pipe several UT techniques may be required. An example of a restriction is the weld cap; this in conjunction with the width of normal incidence transducer precludes colour graphic imaging close to the weld or indeed of the weld itself. The Time of Flight Diffraction (ToFD) technique however enables this area to be reliably assessed. Some geometries of weld do not readily lend themselves to inspection by ToFD. Examples of these are nozzle or penetration welds and these may be inspected and monitored using pulse-echo shear wave ultrasonic techniques.
Corrosion surveys on vessels are in many instances being performed by internal visual surveys or by carrying out ultrasonic thickness measurements from the external surface. Internal visual surveys require the vessel to be opened, cleaned and in many cases vessel internals or catalysts have to be removed. This can therefore only be performed during a plant shutdown. Although corrosion is readily detected, it cannot always be quantified. External ultrasonic surveys may provide quantified results but it must be taken into account that point readings taken at regular intervals over the surface of a vessel represent a very small fraction of the total surface area.

Fig 1: "C" and "B" Scan Image of Corrosion

Fig 2: Composite "C" and "B" scan image of severe corrosion

Fig 3: Composite "C" scan image of a pressure vessel The above image is compiled by stitching individual 500mm X 500mm scans together using a CAD program

Fig 4+5: Above images from top to bottom, 1) "C" and "B" scan from 0º compression wave probe, 2) ToFD "D" scan and 3) "C" and "B" scan images from 45º shear wave probe of Hydrogen Induced Cracking

3. APPROACH TO THE CONTROL AND ASSESSMENT OF SERVICE INDUCED DEGRADATION

3.1 STRUCTURE

Phase 1: Planning:
A written procedure for the examination is prepared after reviewing operational conditions, plant history, nature of product, materials, experiences with similar plant, plant design, consequences of failure, etc. Phase 2: Screening:
The areas highlighted for inspection often constitute a large volume of material and in many instances a large area of operating plant. It is unlikely that it will be economical or physically feasible however, to screen large areas of material using only ultrasonic imaging techniques. The choice of screening techniques is wide, new techniques are continuously under development, with each application requiring individual assessment, examples of screening techniques include:

• Visual survey
• Manual ultrasonic survey
• Magnetic flux leakage
• Thermography
• Coarse resolution automated or semi-automated ultrasonic surveys
• Acoustic emission
• ACPD / ACFM
• Eddy Current or Remote Field Eddy Current
• Pulsed Eddy Current
• SLOFEC
• Guided Wave ultrasonic inspection
• Intelligent pigging - flux leakage or ultrasonic

The initial objective is to identify the presence of material degradation. Only in those cases where this is identified do further, more detailed examinations need to be performed.
Phase 3: Analysis:
The results of phase 2 are analysed to assess the overall extent and severity of degradation if any. These results are used, to plan further inspection around those areas, which have been demonstrated to be the most significantly affected. This may involve expanding the scope of the original inspection and/or re-inspecting relevant areas using higher resolution and more critical techniques. The benefits of advanced computerised ultrasonic techniques are that the extent of material degradation is more reliably reported and in certain circumstances the cause of the problem may be identified.
STEP 4: Propagation monitoring:
Areas analysed during phase 3 can be monitored on a periodic basis to accurately measure the rate at which material degradation occurs. Propagation monitoring is performed on line wherever possible. The screening exercise, under most circumstances, is carried out only once during the operating life of the plant. Subsequent material degradation monitoring is then concentrated on areas where specific degradation is detected, which could threaten the continued safe operation of the plant. The information generated can be used to establish plant remaining life and to assess the implications of continued plant operation as well as providing critical information required to operate a condition based inspection strategy.

3.2 INSPECTION STRATEGY

Ultrasonic colour graphic imaging of plant can be approached in a number of ways. There is no single procedure to suit all situations as circumstances vary from plant to plant and from equipment to equipment. It is generally impractical to produce colour graphic maps of all vessels, nozzles and pipework on a plant. This level of assessment is restricted to those items which are considered to be at risk as a result of prior inspection data or with reference to operating conditions, materials, original design parameters and can be based on the results of a RBI analysis or be the result of a Fitness for Service or Remaining Life Assessment study. The following are examples of inspection programmes currently in field use: a) Governing Requirements:
Most industrial plant around the world is licensed or insured with set time limits on the operation of the plant. This is dependent on the original design and conditional on a shutdown inspection program being observed. Due to prevailing worldwide economic conditions there is very often a requirement to extend the life of plant, beyond its original design life. Hard copy colour graphic images and weld 'profiles' of critical vessels and associated pipework in conjunction with a full RLA analysis may be used as the basis for establishing that the plant may be operated for an extended period within safe limits.
b) Approach:
The RLA analysis and prior inspection results, obtained by visual and conventional NDT inspection surveys are processed, prior to a shutdown to establish which items or parts of items are the most likely to significantly degrade (phase 1). These items are then scanned using a relatively coarse resolution with colour graphic imaging (phase 2) over the parent material. With TOFD being performed on circumferential and longitudinal welds and manual pulse echo on nozzle welds. The resultant data will highlight the most significant areas which are subsequently analysed in detail (phase 3) with evaluation by ToFD, automated PE and high resolution colour graphic imaging and are subsequently monitored (phase 4), the timing for which may be established through a Fitness for Service regime, which could include a critical defect analysis.
c) In-Service Inspection:
One of the most important drawbacks of performing inspections during a shutdown is that in the event that serious degradation is detected, which requires immediate intervention, material, procedures and other resources have to be organised immediately and this very often leads to an extension to the shutdown period. Due to the portability of the ultrasonic inspection system a large portion of the inspection may be able to be carried out via access engineering thus negating the requirement for scaffolding and significantly reducing the associate cost.
High temperature couplant and probes enable scanning to be performed reliably and continuously at surface temperatures up to 250 degrees centigrade. Probes are available for taking occasional readings at higher temperature, but these are unreliable where prolonged contact with the surface is involved. Where significant degradation is detected during in-service inspection, remedial work can be planned and structured prior to the shutdown.

4. WELD INSPECTION

The conventional method of testing welds in pressure systems is the Pulse Echo (PE) technique, which is usually based on the comparison of reflectivity from a known test block. In the late 1970's an alternative ultrasonic technique was developed - ToFD.

Time of Flight Diffraction (ToFD)

The ToFD technique involves using a pair of angled, broadband, compression wave probes, set astride a weld facing each other. One probe acts as transmitter and the other as receiver. The beam and amplifier characteristics are selected to produce as wide and even distribution of ultrasonic energy as possible in the through-wall plane. The first signal received is the upper edge of the beam, referred to as the 'lateral wave'. The signal represents the shortest path between the two probes and on a flat surface this represents the material outer or scanned surface. Part of the beam will be reflected from the inner surface, or back wall of the item. This signal arrives later in time than the lateral wave. With prior knowledge of the material velocity and the physical separation between the two probes, the material thickness can be accurately and reliably calculated. In the event that preferential weld or heat affected zone corrosion/erosion is present, a reflection will be obtained from the upper surface of the corrosion profile, which will arrive at the receiver before the back wall reflection.

Fig 6:

The depth of penetration of this type of degradation can be very accurately measured. The ultrasonic waveform is fully digitised and stored with the data displayed in real-time to create very high-resolution images of weld internal details. All data is collected with the Pocket Scan operating in the ToFD mode and is stored to optical or magnetic disk for future reference.

Fig 7: Single axis, encoded 6 probe scanner

The ToFD technique is also responsive to planar and volumetric buried and surface breaking flaws of all orientations, anywhere in the weld, heat affected zone or adjacent parent material. This means that original manufacturing discontinuities can be monitored for growth during service and service induced flaws such as Stress Corrosion Cracking (SCC), Hydrogen Induced Cracking (HIC), Hot Hydrogen Attack (HHA) and Underclad cracking may be detected and monitored. Manipulation may be performed using dual axis scanners, but the greatest flexibility is obtained using a manually propelled single axis encoded scanner.

Fig 8: ToFD Image of an Underclad Crack in a Heavy Wall Reactor	Fig 9: Image of Stress Corrosion Cracking (SCC) in piping in a Petro-chemical plant

As all waveforms are saved, the off-line software processing possibilities are numerous. An example of this is the ability to carry out very precise propagation monitoring by creating profiles of defect images. These profiles are superimposed over profiles generated from subsequent inspection data to establish whether any propagation has taken place in the intervening period. As the evidence is hard copy it can be used to justify leaving technically rejectable indications in service, as proof is available that the indications are stable and no propagation has occurred.

Pulse Echo Inspection

Nozzles and complex geometries do not readily lend themselves to inspection by ToFD. Although ToFD techniques are now available to carry out these inspections the technique is still embryonic and extremely costly. These may be more effectively inspected using a conventional ultrasonic inspection technique (PE) where reflected signals are evaluated. The inspection can be performed using automated multi axis scanners and sophisticated software however in many instances conventional manual pulse echo ultrasonic testing with correctly skilled operators is adequate.

5. CONDITION ASSESSMENT

5.1 Plant Life Assessment
With accurate, thorough and reliable data on plant condition, plant remaining life can be accurately assessed. Ultrasonic imaging techniques provide this type of data and using this plant remaining life can be more confidently predicted. The implications of errors in remaining plant life calculations are that the plant may be unnecessarily prematurely retired in the case of under assessment, or conversely the plant may be operated in an unsafe and unreliable condition in case of over assessment. This has equal importance to plant remaining life and life extension programmes.

6. ADVANTAGES

The major advantages of the system described are as follows:

• Plant remaining life can be more accurately assessed.
• Plant life extension programs may be undertaken with confidence.
• Safety and reliability are more confidently assured.
• Plant life may be optimised against production.
• Repair/replacement programs may be more reliably planned.
• The inspections can be carried out non-intrusively and in many instances while the plant is on line
• Information on areas requiring repair is available prior to shutdown.

Advanced methods also detect corrosion and other service related problems during their early stages and enable monitoring to be performed during service and Non-Intrusively to predict safe operating periods. In certain instances the cause of the problem may be determined without the necessity of opening the vessel.

7. CONCLUSION

The improvements in inspection technology, which enables early detection and accurate sizing of material degradation, in conjunction with a refined condition management strategy, can provide the information to avoid unscheduled shutdowns and outages and as such offers major improvements in safety and profitability.

Nondestructive Testing Technologies for Local Industries

Nondestructive Testing Technologies for Local Industries

Marwan F. Basrawi
Saudi Aramco
E-7720, Engineering Office Building
Dhahran, Saudi Arabia
Fax: 873 5670

Corresponding Author Contact:
Email: marwan.basrawi@aramco.com

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ABSTRACT

Nondestructive Testing (NDT) is proving itself and industries that are seriously investing in it are realizing millions of dollars per year in cost saving and/or cost avoidance, especially by detecting corrosion before it's too late. However, the most important investment in NDT is in the human resources part of it. NDT is not only about equipment, but even more about the professionals operating the equipment and interpreting the results. Proper training and certification is paramount to this aspect. Each newly developed NDT technique leads to another more effective one that may be adapted for purpose and finally implemented effectively. Effective implementation is achieved only through proper up-to-date training and certification for even the most conventional NDT techniques. Otherwise, high potential NDT technologies are doomed for failure through misapplication and then eventual surplus without the realization of any cost saving, quality and safety benefits. Having stated the proven significance of NDT and its effective implementation, this paper proceeds to present some NDT technologies, from the most conventional to the more advanced, and their proper and appropriate application and resulting benefits. It will also provide direction on successful training and certification of NDT professionals to operate the associated sophisticated equipment.

1. INTRODUCTION

Advanced Nondestructive Testing Technologies are emerging faster than our local industries are realizing and appreciating. Industries need to participate more in their development and implementation to realize more of NDT's benefits. Competition is ever increasing and this obligates reducing production and operation costs. This is where NDT can figure in on a large scale. NDT can save and/or avoid production and operation costs in millions of dollars for our local industries.

2. STATEMENT OF THEORY AND DEFINITION

A general definition of nondestructive testing (NDT) is an examination, test, or evaluation performed on any type of test object without changing or altering that object in any way, in order to determine the absence or presence of conditions or discontinuities that may have an effect on the usefulness or serviceability of that object (1). 1. Hellier, C.J.: Handbook of Nondestructive Evaluation, McGraw-Hill, New York (2001) 1.1

3. DESCRIPTION OF APPLICATION OF EQUIPMENT AND PROCESSES

The test object for NDT is industry equipment, from storage tanks to pressure vessels to heat exchangers and from pipes to valves, etc. And the splendor of it is that it's in-service inspection. Facilities may continue in operation while comprehensive NDT is done. There's no need to shut down the facility for most NDT, and there you have savings already from the start. An excellent example of this is the most passive non-intrusive testing of Acoustic Emission on storage tanks and pressure vessels. This recognized NDT method merely listens for corrosion activity through probes placed around the tank or vessel, without having to empty the storage tank and while the vessel is still charged and in service. The probes are sensitive enough to detect corrosion and the computers are advanced enough to process the location of the corrosion that has an effect on the serviceability of the tank or vessel. However, the sensitive probes and processing computers are only the beginning of this proven NDT method. It takes properly trained and qualified experts to interpret the readings to avoid "garbage in, garbage out" scenarios. Acoustic Emission is just one of the eight basic NDT methods in which technicians may be trained and certified as high as Level III. Training for certification is paramount, since without it the whole NDT system can only fail and the proven methods will not be appreciated in the way they can be. This is where the American Society for Nondestructive Testing has a significant role as not only a proving ground for the ever advancing NDT methods, but this society also develops and manages credible certification schemes for the NDT industry, starting with the most basic of all, Visual Testing. Visual Testing or VT was the first NDT method used but the last method to be formally acknowledged through training and certification programs. Direct visual testing is defined by ASME as using visual aids such as mirrors, telescopes, cameras, or other suitable instruments, when access allows the eye that does the visualizing to be within 25 inches (610 mm) of the surface to be examined, and at an angle not less than 30 degrees to the surface that is to be examined. Remote visual testing is done through borescopes, fiberscopes, and video technology. This goes to show how even in such an obvious method as visual testing, it is important to be properly trained and certified. The list of NDT methods goes on to include Penetrant Testing, Magnetic Particle Testing, Radiographic Testing, Ultrasonic Testing, Eddy current Testing, Thermal Infrared Testing, and finally Acoustic Emission Testing, the method first introduced as a good example of in-process testing and the most passive one at that.
Each of these NDT methods comes along with multitudes of associated equipment, from the most basic tool of a magnifying glass for visual testing to highly advanced computer processors in sophisticated equipment for Acoustic Emission and Thermal Infrared Testing. The more sophisticated equipment requires its own specific training and certification to be operated credibly. If this requirement is not satisfied, it can render NDT ineffective. This would be a direct result of inappropriate or misapplication of the NDT equipment.
Unfortunately, it is possible to buy and sell sophisticated NDT equipment without adequate technical support, be it qualified operators or maintenance. Thus, high potential NDT technologies are procured to be doomed for failure through misapplication and then eventual surplus without the realization of any cost saving, quality and safety benefits.
This leads to the issue of NDT credibility, also a most critical aspect for effective NDT that may be abused. Credible NDT requires third party unhindered expert application. Conventional NDT needs to be monitored closely by the client and advanced NDT should be procured from prequalified third party service providers. Monitoring and prequalification should concentrate on operator qualifications as well as the application of the NDT service being provided. The concerned client can only do this, as NDT certification and application are not normally regulated by any outside authority. ASNT does not do this as a support society, but it does provide comprehensive NDT certification and application programs for the industry to use effectively to realize cost saving and safety benefits. The transient nature of the work force that performs NDT in our region further complicates this credibility issue. Many of the NDT technicians are recruited on a contract basis and some are intimidated into NDT interpretations that suit their clients, be they their employers or their employer's clients. It is as simple as not calling a detected defect so that they may remain employed by not making their clients do costly repairs, thus turning a science into art. This is how NDT's credibility is challenged time and again, casting the element of doubt on the effectiveness of NDT, when defects are eventually realized after NDT did not supposedly detect them as it should have.
On stream Inspection (OSI), Plant & Equipment Integrity, Fitness for Service (FFS) and Risk Based Inspection (RBI) programs are based on nondestructive testing. Most of the input data for these programs comes from NDT, from the most basic to the most advanced methods. In equipment integrity programs, corrosion loops are established that rely on NDT input. A Fitness for Service study on a storage tank is entirely an NDT method unto itself, which is the Accoustic Emission that has been mentioned.

4. SPECIFIC NDT TECHNOLOGIES

Having introduced the significance of NDT and its effective application, here are some proven NDT technologies, from the most conventional to the more advanced, their required training requirements and proper application and resulting benefits. NDT methods can be broken down into two main categories: conventional and advanced. The conventional NDT methods are those that have been in use since as early as the 1950's and have proven to be a daily routine examination for many. These methods include:

• Radiography
• Ultrasonic
• Magnetic Particle
• Liquid Penetrant
• Visual

Radiography Testing (RT) is widely used for both welding examinations during construction and corrosion/erosion detection after facilities are placed in service. This method utilizes a radiation-emitting device, e.g., x-ray or iridium, and industrial film as a recording medium to produce a latent image of the piece being radiographed. Sensitivity in the range of 1% of the material thickness is easily achievable with the latest films available. Issues of Radiation Safety, Operator Training and Radiographic Film Interpretation (RTFI) are all areas that add to the complexity of this NDT method, but when the individuals are trained properly, this tool can be a very cost effective and safe way to inspect material. Initial training requirements for this method start at a minimum of 80 hours of technical training and require a minimum of 1200 hours of recorded hands-on experience in NDT (600 of which must be directly related to radiography). A series of general, specific and practical examinations and documented proof of the minimum requirements are all part of the certification process to become a Level II Radiographic technician. Typical results of the radiographic process are shown in Figure 1.

Fig 1: , Radiograph of Valve

Ultrasonic Testing (UT) is probably the most widely used type of conventional NDT today. Ultrasonic testing is performed to take simple thickness readings of material for on-stream inspection, perform more sophisticated weld examinations for inherent flaws in the welds and performed in both semi-automatic and automatic modes to map corrosion of piping and pressure vessels. Figure 2 is a typical ultrasonic A-Scan. Newer automated and semi-automated devices to handle the high temperature systems in refineries are being produced now to allow on-stream examination of material that is up to 700 degrees C in temperature without having to shut down the process. The requirements for training and experience are very similar to that of RT, but the 600 hours of actual UT experience are required instead of the RT experience.

Fig 2: Typical Ultrasonic A-Scan

The two NDT methods just covered are considered volumetric examinations as they investigate the areas inside the material. There are common types of conventional NDT methods that deal strictly with surface or very near surface defect detection, as follows.
Magnetic Particle (MT) is an electromagnetic examination that is used to locate and estimate the size of defects that are either open to the surface, such as cracks, and delaminations or very near subsurface defects such as gas holes and inclusions. This test uses an AC, DC or permanent magnet to introduce lines of magnetic flux into the material and small visible or fluorescent metal particles that are dusted on the surface. Any disturbance in the lines of flux will cause the metal particles to collect in the area of disturbance and produce an outline of the defect causing the flux line disturbance.
Liquid Penetrant Testing (PT) is one of the oldest conventional NDT methods used today. It was first used as an oil and powder type examination to test railroad car wheels for cracking. The wheels were soaked in light oil (kerosene) and then lightly dusted with powder. The powder would pull out the oil that had seeped into the cracks in the wheels to reveal the location and size of the flaws. Today's liquid penetrant material is much more sophisticated than the old oil and powder method, but still works on the same principals. Highly viscous penetrants (mostly colored to clash with the developer that is applied at the end) are sprayed on the part and allowed to dwell for a specified period. Excess is removed and a developer is applied to draw out the trapped penetrants and reveal the defects locations and size. This method can be used on any nonporous material such as metals, plastics, concrete, etc.
Visual Testing (VT), the most frequently used and most overlooked conventional NDT method, can be very simple and very complex. Simply looking at a part and identifying areas of concern is the simple part. Remote visual inspection using borescopes, fiberscopes and even robotic video devices is the far more complex part and requires significantly more training and expertise. Both types of visual inspection can be used on virtually any material where access is possible.
Over the past few years, other somewhat nonconventional methods of NDT have become more widely accepted as more engineers become more familiar with them to render them to be now considered by many as conventional methods. These include:

• Electromagnetic Testing (ET)
• Acoustic Emission (AE)
• Thermal/Infrared (IR)

Electromagnetic Testing (ET) is one of the oldest forms of NDT, but it was not until recently that this testing method was made readily acceptable with the development of associated user-friendly instruments. Eddy current theory is based on the fundamentals of electricity and magnetism and the inductive properties of alternating current. This NDT method is used commonly in tube testing of non-magnetic materials, production testing of tubing and pipe and it is quite easily used as a tool for material sorting. Training is required and significant field experience is mandatory to enable the technicians to become proficient in the analysis of the ET data. Minimum technical training of 80 hours and 1200 hours of required experience are essential for the proper operations and interpretation of this NDT method.
Acoustic Emission (AE) testing is another NDT method that has recently taken on its important role as a routine inspection method that is being used more frequently in industries around the world. AE is based on elastic energy that is spontaneously released by material when it undergoes deformation. The first evidence of acoustic emission in metals was the detection of "tin cry", a phenomenon of pure tin during plastic deformation. This NDT test method has progressed to being used widely in the examination of storage tanks for tank floor corrosion, pressure vessel examinations for detection of cracking and corrosion and in pipelines under hydrotest to detect weak areas and leaks. AE is capable of detecting very small areas of corrosion when properly applied. Significant training and experience are required for technicians to perform this test effectively. Figure 3 is Acoustic Emission Data from a Storage Tank Floor.

Fig 3: AE Data from Storage Tank Floor

Thermal/Infrared (IR) testing came about in NDT in the early 1960's after considerable testing by the military. Based on the emittence of energy from any material, the infrared radiometer is capable of detecting skin temperature differences in a very small range. Infrared Thermography has become very applicable in many ways due to today's advancements in microprocessor equipment. However, analysis of the IR data is far more complex. Requirements for technical training are currently at a minimum of 80 hours, but 900 hours of practical field experience is required to obtain a level II certification as an IR Thermographer.
As mentioned previously, advancements in NDT technology is growing in leaps and bounds. The remainder of this paper will discuss some of the more notable advancements, most of which are a derivative of the conventional methods we have just discussed.
There is much advancement in NDT, which has proven to be very cost effective and easy to use. However, some of the newer NDT methods are very complex and difficult to use, but produce results that make the complexity of the test worth the time and effort. When one considers the huge sums of money spent each year in shut-downs, cleaning and inspection of plant piping and equipment and the negative environmental impact from waste produced during these cleaning and inspection exercises, research efforts should be focused on developing more advanced NDT methods that can lessen if not eliminate these costly and negative environmental impacts. Many times the results of the costly internal inspections show the effort could have been avoided if viable non-intrusive NDT methods were proven and available.
Some of the newer and more advanced NDT methods that have recently become available for use are:

• Electromagnetic Acoustic Transducers (EMAT)
• Guided Wave Ultrasonic Testing
• High Energy Radiography

Electro-magnetic Acoustic Transducers (EMATs) are a revolutionary method of introducing ultrasound into a part without the normally required liquid couplant. This non-contact method of ultrasonic inspection provides for high-speed examination of piping and pressure vessels. This method of NDT introduces the ultrasound via electro-magnetic coupling, making it very suitable for use at higher temperatures than conventional UT. This also eliminates the need to shut down the component for inspection. With plant or refinery processes running in upwards of 1200 º F temperatures, conventional UT becomes obsolete and EMAT has proven to be a very valuable method to examine in-process piping and pressure vessels to assure safe operation of the components. Technical training in this NDT method is required in excess of the normal electro-magnetic training previously discussed and governing standards have yet to stipulate regulations for the training and experience requirements. However, many industry standards are reviewing it at this time.
Guided Wave Ultrasonic testing is fast becoming the choice of many industries. This NDT method's ability to examine long distances in a single setup (approximately 30 meters in both directions from the transducer ring) on both insulated/coated and buried pipelines makes it very cost effective and it provides an enormous amount of information for condition assessment. The equipment introduces bulk UT waves at somewhat lower frequencies (10 to 25 KHz) than that of conventional UT or EMAT technology and provides a means of detection of anomalies at great distances from the source of the propagating waves. Although a quantitative exam, it has become very useful as a screening tool to identify areas of concern for follow-up examinations with other more exacting and time consuming tools. This NDT method will eliminate costly unburying of lines that were not required and provide substantial costs savings by decreasing the number of digs. Further development is needed to refine this process to allow increased sensitivity and overcome the current problems faced with tape-wrap and heavy coating. These tend to deaden the sound significantly and greatly reduce the length of piping that can be examined at one time. Interpretation of the results from the Guided Wave UT tests is similar to that of conventional UT, but it does require additional training and experience to fully understand and benefit from the results. Figure 4 is a graph of Guided Wave Signals.

Fig 4: Guided Wave Signals

High-energy radiography (2-10 MEV) has been around for quite some time but it just recently made its way to the portability from which especially the geographically widespread oil and gas industries can well benefit. Conventional radiography has thickness limitations of approximately 3 inches of steel or equivalent, unless Cobalt 60 is used which provides very grainy and hard to interpret images. The introduction of portable "Linear Accelerators" has made possible the field radiography of material with thickness of up to 14 inches of steel and with current high output options available on some newer instruments, 20 inches of steel is no problem for the portable accelerators to handle. Given the equivalency factor for oil as compared to steel, the portable linear accelerator can easily be adapted for corrosion surveys of 48-inch diameter oil filled pipelines, examination of small oil filled vessels and huge valves. This is just a few of the many number of situations for which the Linear Accelerator could provide valuable information for plant reliability and safe operations.
The above mentions only a few of the new advanced NDT methods that are being used in industies today. Many others such as, Thermal Wave Imaging, high speed tube testing with Guided Waves, filmless radiography using phosphor imaging plate and others are being introduced almost daily and beneficially implemented by companies to extend their operations and eliminate downtime of process equipment.

5. CONCLUSION

NDT can save and/or avoid costs in millions of dollars for facilities that use its methods. There are proven NDT technologies to do this, from conventional to more advanced ones that are essentially based on the conventional ones. Their required training requirements and proper application are paramount for realizing ever-increasing benefits.

CRITICALITY CLASSIFICATION of Pipelines to ENSURE MECHANICAL INTEGRITY of PLANT & PIPELINES

CRITICALITY CLASSIFICATION of Pipelines to ENSURE MECHANICAL INTEGRITY of PLANT & PIPELINES

A. K. Astana, SHARQ - SABIC
A.M. Al-Zahrani. SHARQ - SABIC

Corresponding Author Contact:
Email:

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SYNOPSIS

Rupture in coolant return line, due to excessive erosion thinning, caused 8 days shutdown. To avoid such incidents, it was decided to comprehensively evaluate Integrity of plant pipelines through a systematic approach to ensure their maximum availability at desired Reliability level. As first step towards building a dependable Risk Based Inspection Program, a plan was made for 'Critical Evaluation' of all pipelines with respect to Safety, Environment, Production & Past History. 3565 pipelines were reviewed individually and 1871 pipelines ( 52 % ) were identified 'High Critical'. Thickness survey of all 'High Critical' pipelines will be completed this year.

INTRODUCTION

This is an attempt to optimize our inspection activities in a scientific approach, to put guidelines for selection of pipelines according to their criticality in either one or more of the safety, production, product quality or pipeline reliability factors. In 1 EG, the 12'' Dia. Coolant Return line of EO Reactor 1101B, ruptured, on 02.12.2000, due to excessive erosion thinning. This caused more than 8 days of unplanned shutdown for 1EG plant. To avoid such incidents in future, it was decided to comprehensively evaluate the integrity of all plant pipelines through a systemic approach so as to ensure their maximum availability at the desired reliability level.
As a first step, SABIC affiliates were contacted to find out practices followed by them and to share their experiences. Review of feedback received indicated, in almost all cases - No study has been done so far. Only pipelines with Problem / Failure history are being monitored at present. In isolated cases, it also included Process Licensor recommendations / lines with Critical process fluid ( eg. Butane - which produces a Vapour Cloud, if leaked to atmosphere ) are being monitored at present.
This indicated that although SABIC affiliate companies carry out pipeline thickness surveys, the selection of pipelines does not depend on any scientific study or clear cut justifiable guidelines. In most of the cases this is based on previous history i.e. Problem lines.
Considering this feedback, SHARQ could have also gone for most simple & obvious method -
i.e. "PIPELINE HISTORY"
But This is a "REACTIVE ACTION" as it does not have any scope for improvement of "PLANT RELIABILITY".
As has been emphasized in the beginning, our aim was to be 'PROACTIVE' and this could be achieved by
"SCIENTIFICALLY ESTABLISHED CRITICALITY CLASSIFICATION AS A FIRST STEP TOWARDS BUILDING A DEPENDABLE RISK BASED INSPECTION / MAINTENANCE PROGRAM IN SHARQ".
This was the reason that SHARQ took initiative to review & classify pipelines as per their CRITICALITY, after discussing with PLANT / ENGINEERING to firm up the guidelines.

METHODOLOGY

As effective programs can not be made for every pipeline in the field considering Cost, Efforts & Time involved, it was decided to review all pipelines in Plants and classify them as per their Criticality. A plan was made for 'CRITICAL EVALUATION' of all pipelines with respect to SAFETY, ENVIRONMENT, PRODUCTIONS (QUANTITY & QUALITY) & PAST HISTORY. It was felt that such scientifically established "CRITICALITY CLASSIFICATION" is of prime necessity and would be first step towards building a dependable RISK BASED INSPECTION / MAINTENANCE PROGRAM and will have great impact on the proper action plans. A team was formed with Reliability - Chief Engineer ( Inspection ) as Group Leader, consisting members from Plant Operation, Process Engineering & Inspection Group to carry out critical review of pipelines. A set of Checklists for identifying Production / Safety Criticality and Failure Probability of Equipment was already available. Each checklist contained a set of 10 Questions. These Checklists were critically reviewed and a set of most appropriate questions were selected and modified to suit the objective. 12 questions representing Safety, Failure Probability and Past History, were prepared to finalize the Check list.
In addition, general Guidelines were also firmed up before starting the activity. Few important points were -

1. As majority of pipelines will fall under 'Production Critical' - this category will not be considered while deciding lines classification / frequency of Inspection.
2. Pipelines below 2" size, will be reviewed on the basis of their service ( eg. - TEAL, Catalyst, Air, Nitrogen etc. ) or if they have a history of problem / failures.
3. Process Fluid was also considered for selection, eg. TEAL lines were taken up based on criticality of process medium.
4. Water lines, specially Sea Water, Cooling Water & Chilled Water lines were considered & reviewed.
5. Pipelines with 2 Phase flow were considered and reviewed.
6. Stainless Steel lines, specially those in Cycle Gas, Steam Condensate service were also considered & reviewed.
7. Pipelines subjected to higher than specified fluid velocity were considered & reviewed.
8. Pipelines in corrosive service were considered & reviewed.
9. Pipelines which have history of corrosion / problem / leak were considered & reviewed.

It was decided to use latest updated P&ID of respective plants, as a reference document for selection and review of pipelines. This way all pipelines could be traced from start to end and were marked. Also it ensured that no pipeline has been missed by mistake.
The methodology concentrated mainly in answering many questions about pipelines in all aspects, where the questionnaire was designed in such a way that the results will enable the reviewers to decide whether the pipeline (as a whole) is High Critical / Medium Critical / Low Critical or Non Critical.
Every individual pipeline was thoroughly reviewed against each question and debated, if required, till a consensus decision / agreement was reached. Final decision was noted down and after completion of complete questionaire, criticality of that pipeline was identified and documented accordingly.
If answer of any question was 'Yes', then only the line was further reviewed to ascertain its Criticality level. Highest criticality rating attained by the lines was considered as its 'Final Criticality'. This activity was taken up on full time basis with the team members exclusively dedicated for this job.
Sample copy of Check list used for this purpose is enclosed. ( Attachment - 1).
As described before, there are four areas and pipelines have been classified accordingly. These are as follows:

1. High Critical Pipelines (Safety wise)
2. Medium Critical Pipelines
3. Low Critical Pipelines
4. Non Critical Pipelines

1. High Critical Pipelines (Safety wise):

For these pipelines, all available and non-available kinds of maintenance, operation care would be exercised. Such application would minimize drastically the chances of sudden unplanned failures and consequently minimize the losses and chances for safety accidents and incidents.

2. Medium & Low Critical Pipelines:

Pipelines that lie in these categories shall mainly be under very close condition monitoring programs, wherever possible. It should also include all NDT practices as well as condition monitoring practices that are usually observed by operation (flow, sound, pressure, efficiency, visual appearance.. etc.). In summary, the ideal case for the Medium & Low Critical pipelines is to be under condition-based maintenance and not time-based maintenance.

3. Non-Critical Pipelines:

These pipelines will be under (Run-to-Fail) inspection philosophy. However, reviewers must be very careful in deciding whether a pipeline is non-critical since this decision will result in ultimately dropping the pipeline from regular scheduled monitoring programs. 3565 Pipelines were reviewed individually and 1871 pipelines (52%) were identified as 'High Critical'.

Table 1

After completion of review, a summary was prepared with all relevant information. Final criticality rating of pipelines, with details, have been put on SHARQ LAN Network with 'Read Only' access
A comprehensive thickness schedule is prepared and a dedicated team has been put to carry out thickness survey of all 'High Critical Pipelines'.
To begin with, all 'High Critical Pipelines' identified, are being thickness surveyed and our Target is to complete this activity by December, 2003. A Master Format has been designed, encompassing all relevant details ( viz. line location, design & operating pressure / temperature, nominal thickness, corrosion allowance, material of construction, date of thickness measurement, minimum / maximum measured thickness values etc ) for each High Critical Pipeline are being entered in the format. Upon completion, all these details will also be put on SHARQ LAN Network for information & reference to all.
SHARQ has already purchased an Inspection Data Management Software 'UltraPIPE' ( by M/s SOS & marketed by M/s Krautkramer, Germany ). All High Critical Pipeline details are being entered into UltraPIPE for the purpose of future scheduling / follow up by the software.
Next step is to review all measured thickness results and devise guidelines to fix inspection frequency for all these pipelines based on their history, corrosion rate and applicable code.
Similar activity is planned for Medium & Low Critical pipelines, in future.
Based on the evaluation, an Inspection & Maintenance programs (PM & PdM) can be developed, which if utilized effectively would greatly contribute to achieve these ultimate objectives.

CONCLUSION

The above discussion reflects our efforts in trying to come-up with an acceptable way in defining the criticality condition. It is of course known that a margin of uncertainty would exist which may require us to make another review as we get more confidence of our approach and after we let the program run for some time and test its results.
It is known that this is not the most ideal case to review only the pipeline as a whole and treat all its parts and component equally but it is believed that the subject proposal is the right step towards applying typical RCM activity in the near future.
The practice can be considered as the first filtration process where a lot of non critical pipelines can be identified and consequently dropped. On the other hand, many critical pipelines not monitored till now, can be identified and put under regular monitoring schedule.
Nevertheless, it is known that more ideas, corrections, improvement can be added to it. But commenting on such proposal should not stop us from proceeding ahead until we reach the real review period where all plants pipelines will be analyzed closely. The result can be expressed in many ways other than the way mentioned here but the background work would always be the same and the results will be very useful in future to optimize SHARQ inspection / maintenance activities.

ACKNOWLEDGEMENT

The authors would like to thank SHARQ Management to support this activity and permission to publish this paper.

A Comprehensive Approach to Reformer Tube Inspection and Assessment

A Comprehensive Approach to Reformer Tube Inspection and Assessment

Brian Shannon, IESCO, Inc.
3445 Kashiwa Street, Torrance, CA 90505 USA
E-Mail: beshannon@iesconde.com
Carl Jaske, CC Technologies
6141 Avery Road, Dublin, OH 43016-8761 USA
E-Mail: cjaske@cctlabs.com

---

ABSTRACT

Hydrogen reformer tube assessment and life predictions require specific inspection and multi-parameter computations to provide plant operators with realistic usable data. A series of non-destructive examinations are used to determine tube condition. These inputs are married with a series of deterministic and finite element calculations for remaining life prediction. A custom software program and inspection technology is outlined in the discussion.

INTRODUCTION

Reformer tubes normally used in the refining, petrochemical and fertilizer industries are manufactured by the centrifugal casting process and heat-resistant austenitic alloys such as HK -40, HP-40, and HP -Niobium modified materials. A design life of 100,000 operating hours has been the normal time-based criteria for considering retirement of tubes. Many operators of furnaces using such tubes desire to change their maintenance philosophy for tube retirement to condition-based assessment rather than time-based assessment. At a cost of several thousands of dollars per tube and a retubing cost of $1MM-$4MM, a significant amount of capital can be inadvertently applied if tubes are retired either too early or too late. There are many reformer furnaces remaining in service beyond the 100,000 operating hours criteria.
Metallurgical examination of tubes removed from such service has typically indicated carbide agglomeration, but no discernable creep voids or fissures.(1) This provides the opportunity to improve reformer furnace life-cycle value by life-extension of the tubes, using condition-based criteria. Rather than remove tubes from service for sectioning and metallurgical examination at every plant turnaround, it is advantageous to use NDE techniques to screen tube condition for environmental damage such as creep. Operational data required for estimating tube condition by analysis are usually not available. Proper determination of tube condition and its ultimate life requires specific in-situ examinations. The disadvantages in removing tubes from service on a sampling basis to determine tube integrity include:

• Catalyst removal
• Early retirement of serviceable tubes
• Late removal of non-serviceable tubes, impacting turnaround critical path duration if it is found that all the tubes need to be renewed
• Maintenance costs. The advantage of removing tube(s) from service to determine condition include:
• True metallurgical condition of that particular tube is known

However, the condition of the sample tube may or may not be a representative of the total number of tubes in the furnace. For an operating facility to change from a time-based to condition-based philosophy requires confidence in the methods and techniques used to determine tube condition. Extracting tubes at a turnaround close to the end of their design life and subjecting them to metallurgical investigation would appear to be fairly well accepted practice. Some facilities have also embraced the use of certain NDE techniques to trend changes in tubes. The actual technique used is heavily dependent upon the following:

• Costs
• Individual plant preferences (limited knowledge of technologies)
• Historical experiences at the specific location
• Turnaround duration
• Availability of analyzed data from reformer tube testing
• Knowledge of the different NDE technologies (strengths and weaknesses)
• Availability of specialist services

To reduce the occurrences of furnace tube removal for condition-based assessment and to improve overall reliability of tube life, the use of NDE techniques on a regular basis during reformer furnace turnarounds is beneficial. The condition of a reformer tube is inferred from the response of a NDE sensor to a change in material properties. As such, there are certain limits on detectability, sizing and characterization of flaws that are heavily dependent on the overall test system characteristics, comprised of the environment, instrumentation, sensor, material under test and, of course, the operator.

DISCUSSION

Reformer tube condition can currently be inferred in-situ by qualitative NDE assessment using the following techniques:

• Diametrical Growth (diameter change with creep in some cases)
• Wall Thickness Measurement (apparent decrease in wall thickness with creep)
• Replication (final stages of creep damage; i.e., macrocracking)
• Radiography (final stages of creep damage; i.e., macrocracking)
• Eddy Current (responds to chromium migration due to overheating and conductivity changes)
• Ultrasonic (responds to attenuation and scattering)

DIAMETRICAL GROWTH

The principal rationale behind this technique is that, as creep damage occurs, the tube bulges. Each material type has its own nominal value of diameter change where creep is considered to have occurred. The following rules of thumb have been reported by various operators over the years. As an example:

• HK-40 -- 2-3%
• HP-45 -- 5-7%

Yet, recent findings show that in some cases, significant growth may be apparent, but the tube may show the absence of internal damage.(1) Using diametrical growth (O.D. and I.D.) may provide a very general indication of tube condition; however, using diametrical growth as a stand alone method for measuring creep damage, or lack of damage as the case may be, may lead to a significant false call on the actual condition of the tube. The issue is further complicated by the fact that no tolerance is given by the manufacturer for tube O.D. measurement; and the tube I.D., while machined, can vary greatly over the length of the tube segment. In fact, the machining process may produce a given I.D. dimension, but because of the variation in the machining process, the tube may see a significant reduction in wall thickness on one side of the tube while having an abundance of material. on the other. While four different samples from the same tube (Figure 1, 2, 3 and 4) had significant changes in creep damage, it is only when the tube reached macrocracking that a noticeable change in the O.D. or I.D. dimension occurred (Figure 5A).

Figure 1 Macrocracking -- Severe Damage	Figure 2 Aligned Voids -- Moderate Damage
Figure 3 Isolated Voids -- Slight Damage	Figure 4 As Cast -- Sound Material

Fig 5A

Fig 6: Diametrical Growth Comparison

Fig 7: HP Modified - As Cast (Unfired) HP Modified - 6% Creep (Fired)

The above scenario is not always the case, as is demonstrated in Figure 6. These tube segments represent fired and unfired samples from the same tube. Significant diametrical growth (6%) is noted at both the O.D. and the I.D., well within the guidelines for tube replacement. Note the total degree of damage is much less than expected (Figure 7). Isolated and aligned voids extend approximately 60% through the wall thickness. Only through the application of other techniques was the true condition of the tube determined.
To assess diametrical growth, manual strapping of the tube is often performed, and the results are tabulated per tube, at specific locations on the tube (normally at burner locations). As this technique tends to be tedious, time-consuming and requires scaffolding, automated techniques have been developed. Current automated techniques include eddy current proximity sensors and displacement sensors.

The 'H' SCAN ׂ displacement sensor is attached to a scanning head that traverses an in-situ tube and records the diameter measurement at pre-determined intervals indicating the precise location of suspect diameter changes. The output of the tool is input directly into the software spreadsheet for data recording and analysis. A typical finished chart is shown in Figure 5B, note the difference in O.D. measurements of the three tube segments. This is a result of the manufacturing variations. Due to these variations, it is preferable if baseline data can be obtained on the tubes when initially installed so accurate trends may be developed.

Fig 5B

WALL THICKNESS MEASUREMENT

As creep damage occurs, an apparent decrease in wall thickness is evident. As an example, average wall thickness measurements were obtained from a tube that had been sectioned at 0.4m, 1.0m, 7.0m, and 11.0m positions; the metallographic condition is depicted in Figures 1, 2, 3, and 4, respectively.(2) There is an apparent decrease in wall thickness for these four sections of tubes, as shown in the graph of average wall thickness in Figure 8.

Fig 8

REPLICATION

Replication is useful for in-situ assessment of reformer tube outside surfaces, to detect overheating that causes microstructural changes. Replication is a "spot" type assessment and is normally used as a supplemental technique. Only the advanced stages of creep damage can be assessed utilizing in-situ replication.

RADIOGRAPHY

Random radiographic examination is normally used as a supplementary technique to confirm the presence of severe cases of creep damage. It is reasonable to expect to locate such damage when it has extended 50% in the thru-wall direction, when the tubes are filled with catalyst and isotopes are used instead of an X-ray tube. Although using an X-ray tube provides an improved quality image, it is not normally employed, because of practical conditions on site.

EDDY CURRENT

Eddy Current techniques have been used for a number of years on HK-40 and HP-45 tubes. The basic principles of the technique can be found in Reference 3. The technique relies on changes in electric circuit conditions; the circuit being the instrumentation, cables, sensing coil, and the item under test. As the mechanical properties of the test materials change, a change in overall circuit impedance occurs, which is displayed on an oscilloscope. By monitoring these changes, it can be inferred that creep damage is present, based on observation of the signal parameters in comparison to similar changes that occurred on known creep-damaged materials. The depth of penetration of eddy currents is primarily influenced by frequency, conductivity, and relative permeability.
Eddy Current coil design is important to obtain adequate sensitivity and signal to noise ratio. Some tubes, such as HP-40 and similar materials that have a high percentage of nickel, require the use of magnetically shielded or biased coils to reduce the effects of material permeability variations. This improves the signal to noise ratio so a reliable test result is obtained, allowing adequate discrimination of creep damage from general material property characteristics.
Referring again to Figures 1, 2, 3, and 4 that depict varying degrees of damage within a reformer tube, the eddy current responses to these samples are as shown in Figure 9. Note the differences in response to the various stages of damage. The eddy current operator evaluates these changes in signal response. Other factors that the operator considers are:

• Varying lift-off, influencing the signal response, scale and welds being typical examples
• Overheating that causes chromium migration, scale formation, and a significant eddy current response in terms of phase and amplitude changes [4]
• Variations in material permeability

Fig 9

ULTRASONIC

Ultrasonic techniques utilized for the detection and estimation of creep damage include:

• Through transmission ultrasonic attenuation
• Ultrasonic scattering techniques [5,6]

Fig 10

The through transmission technique is shown in Figure 10. The basis is a pitch-catch technique, and it relies on ultrasonic attenuating and scattering due to the presence of creep voids and fissures. The amount of scattering is assumed to be a function of the amount of damage present. Referring again to Figures 1, 2, 3, and 4, that depict varying degrees of damage, the images outlined in Figure 11 depict the four samples and their responses to the ultrasonic examination. The primary disadvantage of this technique is the influence of tube surface condition, which can vary from smooth, dimpled, tightly-adhering scale, to loose scale, or a combination of them all, that affects the ultrasonic signal and gives the impression of creep damage. This can be clearly demonstrated by referring to the two samples outlined in Figure 6, which depict as-cast and fired samples from the same tube. Figures 12 and 13 display the response from the ultrasonic attenuation technique; however, the response from the fired coupon would indicate much less damage than the new or as-cast coupon. This is caused by the signal attenuation due to the surface condition of the as-cast tube. Careful evaluation of a suitable ultrasonic technique is required to demonstrate its suitability for the examination of cast materials. Using an incorrect ultrasonic attenuation technique as a stand alone assessment tool in this case could lead to a significant false call.

Fig 10

Fig 12 (As Cast) Fig 13 (Fired)

The scattering based ultrasonic scattering techniques is similar in principle to the ultrasonic backscatter technique used in High Temperature Hydrogen Attack (HTHA) evaluations in terms of the signal shape, amplitude and location of the signal response(6) that the operator evaluates in comparison to signal response from a "sound" section of tube. The ultrasonic backscatter technique used for HTHA determination relies primarily on signal features such as amplitude, shape, and location. The scattering technique utilizes similar signal features. Figure 14 illustrates a "sound” section of tube, and Figure 15 shows a damaged section of tube. Note the high amplitude signals with length. This technique indicates when surface conditions influence the ultrasonic signal.

Fig 14

Fig 15

COMBINING NDE TECHNIQUES ¾ 'H' SCAN ׂ TECHNOLOGY

Review of the NDE techniques outlined above illustrates some of the advantages and disadvantages associated with each individual technique. Extensive trials have been conducted to determine the viability and optimization of the various techniques. It is currently concluded that no one technique can in all cases provide stand-alone information that will allow complete quantitative assessment of tube condition.(2,13) It is therefore prudent to combine NDE techniques to improve the overall reliability of reformer tube condition evaluation. The optimum combination of NDE techniques is dependent on:

• Type of material
• Type of suspected damage
• Surface condition of material
• Time frame allowed for data analysis
• Cost

The one common element in obtaining NDE data is use of a powered carrier mechanism that traverses the length of a tube. The following NDE sensors can be loaded onto a carrier mechanism for simultaneous data collection:

• Ultrasonic (attenuation, scattering and wall thickness)
• Eddy Current
• Profilometry

Fig 16

Figure 16 shows the IESCO 'H' SCAN ׂ assembly of carrier and sensors. It takes about 1 hour to set up such a system on-site and 2-4 minutes per tube for data collection and to assign a provisional condition status. The NDE specialists evaluate each tube and assign a damage grade per tube determined on the worst section of tube. These grades are assigned based on comparison of each tube to the NDE responses obtained from samples subjected to metallography at the IESCO facility.

TUBE LIFE PREDICTION

Analytical methods can be used to predict reformer tube creep life. The most commonly used method of creep life prediction is based on calculating stress using a simple formula and characterizing material stress-rupture data using a time-temperature parameter. Advanced methods are sometimes used to predict tube life. They use finite element stress analysis and detailed modeling of both material creep and stress-rupture behavior. The most well-known and widely used of the advanced techniques is the pcTUBE™ computer program(14-16) that was specially developed for reformer tube creep life prediction. This section of the paper reviews these methods of tube life prediction and then discusses a new method of directly coupling the advanced life prediction techniques with the inspection results to produce an integrated assessment of remaining tube life. Figure 16.

DESIGN LIFE PREDICTION

Fig 17: Window Rupture of a Catalyst Tube Cracked Catalyst Tube.

As pointed out previously, reformer furnace tubes are typically designed for a minimum life of 100,000 hours. The design objective is to avoid creep-rupture failures, such as those shown in Figure 17. Design tube life normally is calculated using the methods of API STD 530(17) or similar proprietary company procedures. API STD 530 contains data for only for wrought alloys and the cast HK-40 alloy. It has no data for the HP or other cast alloys. These data are plots of stress versus the Larson-Miller parameter, as shown by the example in Figure 18. The Larson-Miller parameter (LMP) is defined by the following expression:
LMP = T (log tr + C) (1) T is the absolute temperature (÷K), tr is the time to rupture (hours), and C is the Larson-Miller constant. The value of C for each material is determined by fitting of the stress-rupture data for that material. API STD 530 does not contain creep deformation data, which is required finite element stress analysis. Stress (S) is calculated using the mean-diameter formula, as follows:
(2) P is the internal pressure in the tube, Do is the tube outside diameter (O.D.), D´i is the tube inside diameter (I.D.), and t is the tube wall thickness. The stress given by Equation (2) is only the pressure-induced mean hoop stress. The variation of stress through the tube wall, the thermal stress, the axial stress, and the effects of cyclic operation are not taken into account by this simplified method.

Fig 18

For the tube design temperature and life, Equation (1) is used to compute a value of LMP. Then, Figure 18 or a similar plot for another material is used to find a corresponding value of S for that value of LMP. Finally, Equation (2) is used to size the tube for the design internal pressure, incorporating an appropriate corrosion allowance into the wall thickness. Depending on the relation of the operating pressure and temperature to the design values and the amount and severity of cyclic furnace operation, this method may produce either conservative or non-conservative predictions of actual tube life.

LIFE PREDICTION USING pcTUBE™

Because of the limitations of the design life prediction method, a special-purpose computer program named pcTUBE™ was developed.(18) The program predicts local material damage related to internal pressure, operating temperature, thermal stress gradient, and cyclic operation.(14-15) It includes material properties, creep deformation models, and creep damage relations for three cast heat-resistant alloys – HK-40, HP-50, and Nb-modified HP.(16) The pcTUBE™ program computes stresses caused by pressure, thermal, and axial loading using the elastic-creep finite element analysis (FEA). The FEA model is illustrated in Figure 19. A segment of the tube wall can be divided into as many as 12 elements. Using such a segment of the tube wall is reasonable because the effects circumferential thermal gradients on hoop stress are not significant.(14-15) The accumulation of creep damage in each element is computed during simulated long-term operation. Important cyclic operations, such as start-ups/shutdowns and operating trips, are modeled using a table of input parameters. Stress relaxation and redistribution during steady operation are modeled. Upon initial loading, a through-wall thermal stress is produced by the thermal gradient through the wall. Creep causes the thermal stress to relax and redistribute with time. The repeated process of application of thermal stress followed by stress relaxation and redistribution are modeled during start-ups/shutdowns and operating trips.

Ad Info Fig 19

As mentioned previously, pcTUBE™ contains data for only the cast HK-40, HP -50, and Nb-modified HP alloys. Thus, it cannot be used for wrought materials or other cast alloys. It does contain an adjustment factor that can be used for a material that behaves similar to one of those included in the program. It accounts for slightly increased or decreased creep strength. For example, this adjustment factor has been used to model the behavior of creep-damaged materials or micro-alloyed HP materials.
The pcTUBE™ computer program correctly predicts that the maximum amount of creep damage will develop between the inner surface and mid-wall of a tube, as shown by the example in Figure 20. It also correctly predicts that tube life is significantly reduced by relatively small changes in operating temperature or by typical cyclic operation. Tube life is reduced by 50% or more for a 20 to 30÷C increase in maximum operating temperature. Two to four start/stop cycles per year may decrease tube life by 50% or more compared with the ideal case of continuous operation with no shutdowns. Thus, pcTUBE™ has been found to be a valuable tool for predicting the service life of reformer furnace tubes.

COUPLING LIFE PREDICTION WITH INSPECTION RESULTS

Operators of reformer furnaces wish to predict the remaining life of in-service tubes. Before the remaining life of the tubes can be calculated, their current condition must be determined. The current condition either can be measured by some destructive or nondestructive test method or calculated using an analytical model, such as the pcTUBE™ computer program. The major drawback of the latter approach is that the uncertainty of knowing the past operating conditions leads to a large uncertainty in analytically predicting the current condition of the tube material. Therefore, it is preferable to measure the current tube material condition and just use the analytical model to predict the future tube life. To provide reliable predictions of remaining tube life, the calculation model used in pcTUBE™ is combined with the results of the 'H' SCAN ׂ inspection. Creep damage and remaining life is computed for each tube in the furnace using inspection results and anticipated operating conditions. The initial damage state of each element (see Figure 19) in the analytical model is set using the inspection data. This approach closely approximates the manner in which damage develops during actual service. The diameter and wall thickness of the tube model also are set using the inspection data. Anticipated operating conditions used in the model include the number, duration and type cycles, outer surface tube temperature and heat flux along the tube, and internal pressure in the tube. Therefore, inspection results are combined with expected operating conditions to predict remaining life.

Fig 20

Fig 21

As is illustrated in Figure 21, which shows a plan view of a reformer furnace, the remaining life distribution is determined. Remaining life is predicted for each tube based on realistic inspection and operating data. These predictions are directly coupled with the inspection data to provide an integrated analysis of the furnace tubes. The remaining creep rupture life of each tube is estimated on a realistic basis, taking into account both NDE measurements on the tube and anticipated operating conditions. The calculations are being incorporated into WinTUBE™ software, so results are available shortly after the inspection and in a timely fashion for decision making by the furnace operator. In addition to making predictions for just the anticipated operating conditions, the potential effects of alternative operating scenarios can be rapidly evaluated.

CONCLUSION

Tube condition cannot be determined by one stand-alone technique, as the degree of damage within a particular tube may not lend itself to that specific NDE technique. The reliability of NDE evaluation of reformer furnace tube condition can be improved by combining a variety of advanced NDE techniques ('H' SCAN ׂ Technology) that individually monitor differing physical parameters. The advantages and disadvantages of each technique, when compared against each other, reduces the occurrence of false calls, improves tube condition assessment and can increase overall furnace reliability. Tube lives predicted using simple design methods do not reflect actual operating lives. Specialized stress analysis and life prediction models, such as the pcTUBE™ software, provide realistic service life predictions. These specialized models are not well suited to computing the condition of tubes in service unless past operating conditions are known very well, which is typically not the case. Coupling specialized analytical models, such as the WinTUBE™ software, with NDE results provides a realistic prediction of remaining tube life.

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